In new residential construction, most houses are built on a concrete slab foundation or on foundation walls. Typically, anchor or foundation bolts are placed in the wet concrete a minimum of six feet on center around at least the perimeter of the house. A wood sill is then used in conjunction with the anchor bolts to tie the house to the foundation by having the anchor bolts pass through corresponding openings through the wood sill.
In order to mark the location of the anchor bolts on the wood sill, the framer normally lays the wood sill on top of the bolts and hits the wood with the hammer to leave an imprint in the wood at the position of the anchor bolts. The framer will then drill the holes, build the wall section and hoist it over the bolts flush with the concrete. In accordance with model building codes, the holes should be no more than 1/16 of an inch larger than the bolts. In practice, framers often drill oversized holes so that it is easier to locate the wall on the bolts and then slam it down flush to the concrete. Then the framer puts a washer and a nut over the bolt and tightens them down to secure the wood sill, thereby securing the wall section to the foundation.
In an earthquake, the bolt to wood sill connection is supposed to restrain the house from shifting off the foundation. However, ground movement causes the anchor bolt to transfer the load to one edge of the hole thereby crushing the wood which could ultimately cause the wood to split. In addition, if the hole is oversized, the house may shift as much as 1/4" before the bolt begins to restrain movement but the house has already built up substantial momentum before it meets with the resisting side of the hole. Therefore, considerable damage can occur above and below the floor line even at very low quake levels. As a specific example, in a recent earthquake in the Big Bear area of California, there were a number of failures in newly constructed homes with oversized holes around the anchor bolts. Some of these homes slipped completely off their foundations and were a total loss.